1. Field of the Invention
The invention relates to an improved power transmission belt.
It relates in particular to such a trapezoidal cross section cog belt adapted for use as endless power tranmission member between two rotary members, one of which is integral with a driven shaft and the other with a driving shaft, for example, in a motor car gearbox or mechanical variator assemblies.
2. Description of the Prior Art
It has already been proposed to form such belts with a body of elastomeric material reinforced with a tension resisting girdle of textile fiber or metal wire cables or cords and, when special characteristics of flexibility of the belt are desired or in certain constructions provided for small winding diameters, transverse cogs on the internal surface, namely the one closest to the axes of the rotary members.
Such known belts, which are manufactured either by injection or by assembling together elastomer layers, cords and rubber coated fabric must, in use, satisfy requirements sometimes contradictory. They must in particular withstand different stress conditions for considerable periods of time, have a high modulus of elasticity under traction, a low modulus of elasticity under flexion and a high transverse mechanical strength. This latter condition is particularly important since, in the absence of sufficient transverse rigidity, the belt may curve during operation, and its cross section may be warped under the pressure of the flanges of the pulleys with which it cooperates and thus cease to play its power transmission role.
These phenomena are further accentuated in devices which require the transmission of a high power, for example greater than 10 kilowatts or so.
To attempt to solve the problem raised, which is that of transmission of a high power, while keeping the advantages of usual belt constructions, namely silent operation, good adhesion, good transmission efficiency and a satisfactory lifespan, as well as the good qualities of flexibility, heat resistance, damping of vibrations and space saving made possible by the internal transverse cogs, it has already been proposed to increase the transverse rigidity of said belts rather than increase their lateral contact surfaces, which would lead to the same result but is a solution which cannot be adopted for space saving reasons.
In order to obtain the desired increased transverse rigidity, it has already been suggested to add, to the rubber mixture forming the matrix body, short uniformly dispersed fibers oriented for the most part in the transverse direction of the belt so as to obtain an anisotropic mixture having a higher modulus in the transverse direction than in the longitudinal or radial direction. In such a solution, however, the value of the transverse ridigity which may be obtained is limited by the fact that the proportion of short natural or synthetic fibers may not exceed about 30% in volume, beyond which value there rapidly occurs a loss of cohesion of the rubber matrix-fiber assembly.
Therefore, other proposals have been made for increasing the transverse rigidity of such belts and, more particularly, the introduction of reinforcement members formed by layers of twisted textile cords oriented transversely or synthetic or natural fiber fabrics disposed on each side of the layer of the twisted cords forming the tension resisting girdle. Proposals of this type are described in No. FR-A-2 503 305, No. FR-A-2 083 034, No. EP-A-0 060 713, No. FR-A-2 484 037 and No. EP-A-0010990; they do not provide a complete solution either to the problem raised in that they do not give the required transverse rigidity to the belt, even though the reinforcement members are perfectly integrated in the elastomeric material forming the matrix body.
Other reinforcement members, for example those described in No. EP-A-84 702 can be placed in the cogs, that is to say in the transverse direction of the belt. They are formed by flat generally superimposed pieces disposed parallel to the base of the belt and are made from metal or from a synthetic material of a resin type. Such reinforcement members are not entirely satisfactory either because they create crack promoting zones in the cogs of the belt due to their flat shape and in addition promote the appearance of delamination zones at the interfaces of their connections with the elastomer matrix body.
To overcome this disadvantage, No. EP-A-84 702 further proposes reinforcing the cogs of the belt by rigid inserts with solid cross section or tubular cross section. The transverse rigidity required for correct resistance under compression of the belt is then obtained but to the detriment of the longitudinal flexibility which has to exist if the belt is to withstand the flexions to which it is subjected during use without damage, by rupture of the cohesion at the interface of the inserts and the elastomer matrix body.
Furthermore, when the reinforcing members used are tubular profiles with polyhedral cross sections such as squares, triangles, etc . . . the edges of these members corresponding to the apices of the polyhedra, in particular those disposed at the top part of the cogs--i.e. in the vicinity of the cable layer or girdle--induce stress concentration zones which result in damage of the elastomeric material forming the matrix body by fissuring the same with, as a result, a substantial reduction in the lifespan and the characteristics of such belts.
It is a primary object of the invention to provide a trapezoidal cross section cog belt of the type mentioned above, namely in which the transverse rigidity is obtained by means of reinforcing members provided in the cogs of the belt but which does not have the drawbacks of the devices of the prior art.
Another object of the invention is to provide such a cog belt which allows the transmission of high powers without however having its longitudinal flexibility adversely affected.
A further object of the invention is to provide such a belt whose useful life is as long if not longer than those of the prior art belts.
Another object of the invention is also to provide such a belt which may be manufactured without difficulty, and in a way substantially similar to known belts, so that it does not involve the creation of special and expensive tools and/or apparatus.